Astronomers detect ancient hydrogen signal, opening new way to map Universe

Astronomers have directly detected an extremely faint radio signal from hydrogen gas billions of light-years away, marking an important step toward using hydrogen to map the large-scale structure of the Universe.

An international research team led by scientists from the University of Manchester and the University of the Western Cape used South Africa's MeerKAT radio telescope to detect emissions from neutral hydrogen dating back to a time when the Universe was several billion years younger than it is today (9 September).

The findings, published in The Astrophysical Journal Letters, demonstrate the potential of hydrogen intensity mapping, a technique that could allow scientists to survey vast regions of space without having to identify individual galaxies.

Mapping cosmic hydrogen

Neutral hydrogen emits a very weak radio signal known as the 21-centimetre line. As the Universe expands, the signal is stretched to longer wavelengths while travelling through space. Measuring that change enables astronomers to study hydrogen from different periods in cosmic history.

Rather than detecting galaxies individually, hydrogen intensity mapping measures the combined radio emission from hydrogen contained in large numbers of galaxies that are too faint or distant to be separately identified.

This enables researchers to survey enormous volumes of space and build a three-dimensional picture of how matter is distributed across the Universe.

Until now, reliable measurements of hydrogen at such distances generally required radio observations to be combined with information from optical galaxy surveys. The new study is significant because the researchers detected the hydrogen intensity mapping signal using MeerKAT radio data alone.

Signal travelled billions of years

The team analysed around 96 hours of MeerKAT observations and detected the signal from two different periods in cosmic history. The radio emissions had travelled roughly four to five billion years before reaching Earth.

The observations enabled the researchers to trace hydrogen across distances of several million light-years, comparable to the distance between the Milky Way and its neighbouring galaxy, Andromeda.

Lead researcher Dr Sourabh Paul said the direct detection demonstrates that hydrogen intensity mapping is moving closer to becoming a practical tool for cosmological research.

The researchers had to carefully remove or account for foreground emissions, human-made radio-frequency interference and instrumental effects to isolate the extremely weak hydrogen signal.

Professor Santos, another member of the research team, said the analysis was particularly challenging because of the many forms of contamination that can affect such faint measurements.

He noted that the data were collected in 2018, when MeerKAT had only recently begun scientific operations, suggesting that a large amount of existing telescope data could potentially be examined using the same technique.

Potential for galaxy studies

The researchers said the achievement could provide new opportunities to study neutral hydrogen over cosmological distances and better understand how galaxies formed and evolved.

Co-author Dr Zhaoting Chen said neutral hydrogen is crucial to understanding galaxy formation and evolution.

He said intensity mapping allows scientists to study the collective hydrogen signal across large regions of space instead of detecting every galaxy separately, offering a new way to investigate both galaxy evolution and the distribution of matter.

The technique could also become increasingly important for future cosmological surveys, including those planned by the Square Kilometre Array Observatory (SKAO). MeerKAT is serving as a precursor telescope for the SKAO.

Professor Laura Wolz of the University of Manchester said the successful extraction of the signal from observations that were not originally designed for hydrogen intensity mapping was particularly encouraging.

She said the result highlights the scientific value of MeerKAT data and provides a pathway for future observations with the SKAO.

Future observations covering larger areas of the sky for longer periods could allow scientists to create more precise maps of neutral hydrogen.

Such surveys may help researchers understand the evolution of galaxies, examine the influence of dark matter on the cosmic web and reconstruct changes in the Universe over billions of years.